The modern information revolution has led to an ever-increasing demand for data storage systems. As a case in point, CD and DVD disks represent successful high volume data storage technologies. One major advantage of these technologies is that reading or writing of data is accomplished by shining light on the disk so there is no physical contact between the media and the optical head. However, the total storage capacity of these disks is limited by the size of the smallest marks on the surface of the media that can be read by the wavelength of light employed. Many attempts have been made to develop data storage systems with progressively smaller marks. However, the required equipment is prohibitively expensive, and the data access rates tend to be unacceptably slow.
One way to increase the storage capacity of a medium is to record the information depthwise, rather than just on the surface. Suitable techniques for depthwise recording include holography, two-photon optics, and similar methods for illuminating media in three dimensions, with the goal of producing marks in three dimensions, and thereby providing very high data capacity systems.
Photoreactions (e.g., bleaching or photochromicity) of organic dyes have also been used as a means to record optical data, both in a single layer in writeable CD-type media, and depthwise (dissolved in a bulk piece of polymer). However, a large amount of optical power is required in these systems to produce readable marks and therefore the rate of recording with such media is slow. Many photochromic systems tend to fade over time.
Holographic recording has also been achieved by optically induced birefringence in suitable polymers, a process that relies on photo-alignment of the side chains within the polymers. Once again, a large amount of optical power is required, and this process is inefficient and slow. In addition, the fidelity of the recorded information may degrade with time since optically induced orientation tends to relax over time in polymers.
JP 2000-086588 discloses a recording medium using changes in circular dichroism based on the interconversion of chiral norbornadiene and quadricyclane derivatives. However, this technique requires enantiomerically enriched compounds that are difficult to synthesize. Furthermore, this application does not disclose the use of recording sensitizers for photoinduced electron transfer.
U.S. Pat. No. 5,759,721 discloses a holographic recording medium that uses a photopolymerization technique suitable for recording information optically in three dimensions.
There is a problem with this process, however, in that photopolymerization is usually accompanied by shrinkage of the material that is a consequence of the process of forming new chemical bonds among the constituents. Any dimensional changes that occur in the medium on writing a hologram by photopolymerization limit the resolution that can be achieved, and reduce the data capacity of the medium. In addition, photopolymerization generally requires the use of low molecular weight reactants so that media made from these materials tend to be undesirably soft or sticky. Furthermore, the most common method of photopolymerization, free radical polymerization, is subject to interference by atmospheric oxygen that causes undesirable inconsistencies in the process. Furthermore, because photopolymerization is used for forming the physical medium, for subsequent optical recording, and finally for fixing the image, much of the available dynamic range of the material is not available for data recording.
Colvin, et al., U.S. Pat. Nos. 5,874,187; 6,165,648; and 6,512,606 disclose a recording medium in which the matrix is polymerized in situ from an oligomer and monomer combination, but the same monomer is consumed for matrix formation and for recording, thus limiting the dynamic range. Dhar et al., U.S. Pat. No. 6,103,454, disclose a recording medium in which the matrix precursor and the photoactive monomer used for recording polymerize by independent reactions. However, their system does not avoid the polymerization-induced shrinkage common to such materials.
U.S. Pat. No. 6,512,606-Lipson et al. discloses an optical data storage system and method comprising a photopolymer medium having a generally polymerizable monomer, an active binder, a first hologram recording polymerization initiator, and a second data recording polymerization initiator. The monomer is preferably cationic ring-opening monomer.
U.S. Pat. No. 6,103,454-Dhar et al. discloses a photo recording medium formed by mixing a matrix precursor and a photoactive monomer, and curing the mixture to form the matrix in situ. The reaction by which the matrix precursor is polymerized during the cure is independent from the reaction by which the photoactive monomer is polymerized during writing of data.